1. Field of the Invention
The invention relates to a regenerative furnace firing system. More particularly, the invention relates to a regenerative heat transfer system for use in heat treating furnaces and reheating furnaces.
2. Description of the Prior Art
Reheating, forging and Heat treating furnaces are utilized to alter the physical, and sometimes chemical properties of a material. The most common application of such furnaces are metallurgical, although heat treatment is often used with other materials such as glass. These furnaces are employed to heat and chill materials, notably steel, often to extreme temperatures, to achieve a desired result such as hardening or softening of the material. Heating methods include softening for forging or rolling, annealing, case hardening, precipitation strengthening, tempering and quenching.
Because reheating and heat treating furnaces operate at a variety of high temperatures, often in excess of 2000° F., they require an increased amount of energy for their operation. Large quantities of hot flue gasses are produced by heating furnaces. The recovery of some of this heat and its reuse in the heating process results in the reduction of the amount of primary fuel needed to run the system, and therefore increases efficiency. An example of such waste heat recovery is the preheating of the combustion air used to fire the burners.
Typically, waste heat recovery from large furnaces utilizes some type of heat exchanger. A heat exchanger is a device built for the efficient transfer of heat from one medium to a second medium. The media may be separated or may mix with other components of the devoce during the heat exchange process. Heat exchangers are commonly used in heating and refrigeration systems, power plants or chemical plants. A heat exchanger may be utilized to retain the waste heat produced by a heating furnace so that it may be reused to reduce fuel costs.
Gas fired fuel furnaces traditionally employ two types of heat recovery systems. Recuperators generally utilize a metallic heat exchanger and have the ability to preheat combustion air to about 800° F.-1000° F. The preheated air and fuel mixture is continuously adjusted as the furnace heats and cools to allow for the proper air/gas combustion ratio. This ratio is constantly monitored and changed as a result of volume expansion and contraction. Adjustment of the preheated air temperature is mainly controlled by the injection of dilution air into the combustion mix. As a result, recuperator systems work well with furnaces that run at a steady state temperature, for example reheating furnaces, forging temperature furnaces and other types, which operate at higher temperatures for extended periods of time. Recuperators are generally not economically practical with heat treating furnaces that require numerous temperature changes, i.e., temperature ramping and cooling cycles.
Regenerator heat recovery systems are more fuel efficient than recuperators and have the ability to operate with higher temperature furnaces, for example 2000° F. or higher. The airflow through a regenerative heat exchanger is cyclical and periodically changes. Hot exhaust air is directed from the furnace through the regenerator where it heats up a stationary medium. This medium may comprise a metallic or ceramic material. The incoming flow of hot waste air stops and cooler combustion air is then passed over the heated medium, which heats the air before it mixes with combustion gas and is directed to the burners. Current heat treatment technology requires that each furnace burner be connected to a single paired regenerator or regenerates within the burner itself during operation. In the case of single paired regenerators, each burner ceases firing at the time when the flow of preheated combustion air stops and the regenerator receives hot waste air from the furnace. Each regenerator therefore does not supply a continuous supply of preheated combustion air to its dedicated single burner. Other types of regenerative (burner) firing systems simultaneously fire and exhaust through the burner itself. However, these systems are many times not economically practical due the expense of each individual burner and the size of the furnace. Using this cyclic firing of the burners for heat treating often causes non-uniform heating of the furnace and too large of a firing footprint to meet uniformity requirements, an undesirable condition for the heat treatment of metals and other materials. The currently developed regenerators are expensive to install because of the need for a regenerator for each pair of burners and the typical space limitations due to the physical size of such regenerators. The temperature uniformity requirements of treating systems are easier to achieve with a greater number of small burners. The use of multiple burners/regenerator pairs also raises the capital investment costs due to the increased hardware cost per unit. As a result, it is generally cost prohibitive to utilize regenerators with heat treating furnaces because of the need for a large number of smaller burners.
There remains a need, therefore, in the art of heat treating, forging and reheating furnaces for a heat recovery system that utilizes burners and regenerators that do not require firing in pairs and therefore have the ability to utilize a number of small burners that achieve heating uniformity and fuel savings. Specifically, there is a need for a system that uses regenerator heat transfer boxes that are not dedicated to a single burner. Such a system allows for the continuous firing of all burners or the flexibility to pulse burners in various configurations that are not dedicated to a specific hardware arrangement, thus providing greater fuel efficiency and more precise temperature control.